I was in Vietnam last year.
I went to Vietnam last year.
With "last year", you MUST use the past tense.
Use "have been" with UNFINISHED TIME-FRAMES.
Last year = FINISHED time frame, therefor past tense.
This year = UNFINISHED time frame, therefor present perfect:
Example: I haven't been to Vietnam this year.
Note** Your lifetime = UNFINISHED time frame, thus:
I have never been to Vietnam. I've been to Thailand and Cambodia, but I haven't been to Vietnam.
I hope that is clear.